12.1 link: Make a hard link via the link syscall

link creates a single hard link at a time.
It is a minimalist interface to the system-provided
link function. See Hard Links in The GNU C Library Reference Manual.
It avoids the bells and whistles of the more commonly-used
ln command (see ln invocation).
Synopsis:

link filenamelinkname

filename must specify an existing file, and linkname
must specify a nonexistent entry in an existing directory.
link simply calls link (filename, linkname)
to create the link.

On a GNU system, this command acts like ‘ln --directory--no-target-directoryfilenamelinkname’. However, the
--directory and --no-target-directory options are
not specified by POSIX, and the link command is
more portable in practice.

If filename is a symbolic link, it is unspecified whether
linkname will be a hard link to the symbolic link or to the
target of the symbolic link. Use ln -P or ln -L
to specify which behavior is desired.

An exit status of zero indicates success,
and a nonzero value indicates failure.